1. Field of the Invention
This invention relates to the field of telecommunications, and more particularly to a protocol for high-speed broadband data communication.
2. Description of the Related Art
Digital Subscriber Line, or DSL, protocols are well-known protocols for delivering relatively high-bandwidth transmissions over telephone lines. There are a variety of DSL protocols, which as a group are often designated xDSL protocols. The xDSL protocols include, for example, Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), High Speed Digital Subscriber Line (HDSL), Symmetric Digital Subscriber Line (SDSL), Multirate Symmetric Digital Subscriber Line (MSDSL), G.Lite (ADSL limited to 1.5 Mbits/sec with a built in POTS splitter) and Very High Speed Digital Subscriber Line (VDSL). Embodiments of DSL systems are disclosed in a standard of the American National Standards Institute, Inc. (ANSI), namely, ANSI National Standard T1.413, Network and Customer Installation Interfaces—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface, published by American National Standards Institute, Inc. All versions of ANSI Standard T1.413 are hereby incorporated by reference. Further information about DSL systems, including ADSL and VDSL, can be found in a variety of books on the subject, including ADSL/VDSL Principles: A Practical and Precise Study of Asynmetric Digital Subscriber Lines and Very High Speed Digital Subscriber Lines, by Dennis J. Rauschmayer, MacMillan Technical Publishing USA, copyright 1999, which is also hereby incorporated by reference.
Two competing technologies for xDSL transmissions are Carrierless Amplitude/Phase (CAP) and Discrete Multitone (DMT). Both of these variants are based on a coding technique known as Quadrature Amplitude and Phase (QAM).
The current format of DSL is based upon DMT which is a form of Frequency Division Multiplexing (FDM). That is, there are multiple frequency cells which are combined to create a single, complex output. The modulation technique that is at the heart of this design is QAM uses a phase change and an associated amplitude to create the bit structure for DMT.
QAM combines variations in signal amplitude with an assigned phase. In the example that follows, phase and amplitude vary simultaneously over four values allowing each cycle to represent one of 16 discrete logical states. This allows 4 bits of data for a single wave cycle or 4 bits/Hz. Referring to FIG. 1, a 16 point constellation 100 is shown, which is often referred to as 16-QAM. The 16-QAM constellation 100 includes an in-phase axis 102 and a quadrature axis 104. The value of the in-phase axis 102 is often referred to as an “I” component and the value of the quadrature axis 104 as a “Q” component by those in the art.
The constellation 100 identifies a signal that is 4 bits/symbol. A chip set in a current design QAM system, be it CAP or DMT, generally uses 256-QAM to increase the number of bits/symbol to 8. FIG. 2 depicts a constellation of the second quadrant 200 of a 256-QAM system.
The combination of amplitude and phase values reflected by a given point in the QAM constellation can be used to render data bits. A QAM system translates the QAM amplitudes and phases into data bits by a mechanism such as a truth table. Referring to FIG. 3, a truth table 300 is shown for an embodiment of a 16-QAM system.
In QAM systems the lowest amplitudes produce lower signal to noise ratios (SNRs) as the line length increases. That is, as line attenuation increases, signal to noise ratio decreases. One problem with QAM systems is that varying signal amplitude (Amplitude Modulation, or AM) is susceptible to external interference.
Referring to FIG. 4, a schematic 400 shows a basic QAM modulator design. A data stream 402 is fed to an encoder 404 and split into two separate streams 405, 406. The streams 405, 406 are fed into two low pass filters 408, 410. Then the streams are modulated onto a sine component 412 and a cosine component 414 of a carrier frequency 418. This separation is the basis for forming the bitstream and realizes 4 bits/Hz. 16-QAM requires a signal to noise ratio (SNR) of 21.5 dB without error correction to maintain a bit error rate (BER) of 10−7. The two streams (which represent the in-phase and quadrature branches of the QAM signal) are added together to produce the composite QAM signal 420. The system can include various other components, such as shaping filters 422, 424.
There are a number of factors that affect the purity of a transmitted QAM signal. There is Gaussian white noise that is associated with all analog signals. When the noise is equal to or greater than the signal, the signal (for all practical purposes) becomes undetectable. This “noise floor” is what all transmitted signals need to exceed in order to be resolved at the receiving end. The question becomes, how much greater than the noise does a signal need be in order to be virtually error free? As stated above, a 16-QAM signal must be 21.5 dB above the noise floor to be correct at the receiving end in order to be virtually error free. (For all practical purposes, a BER of 10−7 represents error free performance.)
Another factor is external interference. There are many forms of external interference including cross-talk (where the signal from one of a pair of adjacent wires interferes with the signal from the other); bridge taps in the line, causing a notch at a narrow band of frequencies; and external interference such as impulse noise from phones going on and off hook. Also, there may be radio frequency interference (RFI) from nearby radio stations or other RF sources.
All of the above factors and others can cause errors in the bit stream, which must either be corrected at the receiving end or, at least recognized, signaling a need for retransmission of the errant message. As described later, many of these problems can be improved via the use of error correction algorithms.
Amplitude Modulation systems inherently depend on accurate recognition of varying amplitudes. As a result, AM systems are very susceptible to all of the interfering factors described above. This particular weakness of all QAM systems will be addressed in the invention disclosed below.
As mentioned above, most CAP or DMT systems use 256-QAM to increase the number of bits per transmitted symbol. The bit structure for 256-QAM is much denser and the bits are closer together than 16-QAM, allowing an interfering signal to more easily corrupt this structure. 256-QAM is also more susceptible to interference from wave to wave. This distortion, known as “Intersymbol Interference” (ISI), occurs when a band-limited channel spreads a channel response to an input symbol beyond its duration period. Tails of previous channel responses interfere with a current one, changing the amplitude of a received signal at re-sampling time. If the amount of ISI is high enough, the receiver threshold logic will read an incorrect value. Once again, the weakness of QAM, AM susceptibility, is problematic for overcoming ISI. The second quadrant block of the 256-QAM constellation 200 shows the close proximity of the symbols in 256 QAM systems.
A 256-QAM system can produce 8 bits/symbol, which corresponds to 8 Mbits/sec when the carrier frequency is 1 MHz. 256-QAM requires a 33.5 dB SNR to maintain a BER of 10−7 when the data rate is 8 Mbits/sec, or 16 times the SNR of 16-QAM. Since an unmodified QAM system could not maintain a SNR of 33.5 dB for very long cable runs, or in the presence of interference, various techniques are used to attempt to keep the SNR as high as possible. Certain types of techniques are discussed below in conjunction with a discussion of two forms of QAM, namely, CAP and DMT Modulation.
Referring to FIG. 5, the encoding scheme of CAP is equivalent to QAM. In this example the orthogonal signal modulation is done digitally by a state encoder 502, in-phase filter 504 and quadrature filter 508. A Digital to Analog Converter (DAC) 510 converts the signal to analog before transmission. This approach allows the QAM technique to be implemented digitally and only the output remains an analog function. Of course, this is CAP in a simplified form. In real applications, CAP is rate-adaptive to conform to varying noise and interference conditions.
The “Web Proforum” group, an arm of the ADSL Forum, ran tests on CAP Rate Adaptive Digital Subscriber Line (RADSL) in the presence of various interfering sources. They found that a 272 kb/sec upstream signal in the presence of a T1 signal is degraded by 15%. That is, if the signal were on a 26 AWG twisted pair phone line and was reaching 26 kfeet, in the presence of T1 interference it would only reach 22 kfeet. Worst case for CAP RADSL was a 784 kb/sec SDSL signal. In this case, the signal reach is degraded by 54% or, reduced from 26 kfeet to 12 kfeet. This degradation translates to a rate reduction in the upstream rate that may be unacceptable for the user. In fact, since CAP RADSL adapts its rate in 300 kbits/sec steps, the rate drop may make this line unusable.
DMT also uses a type of QAM modulation but the implementation is very different from CAP. CAP operates with a single frequency, while DMT operates with multiple frequencies. A given CAP symbol only lasts for a short time, but over a very wide bandwidth. Each DMT symbol lasts for 250 microseconds but is very narrow in bandwidth (4 kHz).
DMT is subject to an ANSI standard, T1.413. This standard describes the constituent parts required to design a DMT system. It also includes a test portion where various line lengths and taps are defined. This allows manufacturers to design chip sets based upon a standard so that some compatibility between vendors is possible.
Referring to FIG. 6, a block diagram is provided for a simple DMT Modulator 600. It is more complicated than a CAP RADSL design, but because it uses QAM as its basic modulation scheme, it is similar to CAP. There are, however, some advantages of using DMT over CAP.
Each CAP symbol transmitted takes all of the channel bandwidth; therefore, a high-level noise impulse, (noise in the time domain) and high-level frequency domain noise (noise present for a long time at a narrow frequency range) will both cause errors in CAP systems. With DMT, frequency domain noise is somewhat avoided, and, since the data rates per channel are lower and symbols longer, the effects of time domain noise are reduced.
In FIG. 6, a Reed-Solomon (RS) forward error correction (FEC) coder 602, 604 or RS Coder, is provided for each branch of the split data stream 608, 610, which is split by a MUX 612. In RS coding, the data stream is broken up into blocks and redundant data is added to the blocks. During processing, a series of finite field adds and multiplies results in each data register (in the processor) containing one check symbol after the entire input data stream has been entered. This encoding scheme must then be decoded at the receiving end.
Decoding the RS coded data stream is a multi-step process. A known field polynomial is used as the basis of creating the RS coding. If there are no errors in the data stream, a comparison to this known polynomial will result in an output of zero. If there are errors, the resulting polynomial is passed to the Euclid algorithm, where the factors of the remainder are determined. This result is then evaluated for each of the incoming symbols (over many iterations) and errors that are found are corrected. This technique provides 3 to 5 dB of improvement in SNR. If there are more errors than can be corrected, then the received codeword is output and a flag is set indicating that error correction has failed. This flag initiates a request to re-transmit the codeword.
The DMT Modulator 600 may optionally include an interleaver 614 for interleaving data of one of the data streams 610. In addition to RS Coding to restore bit errors, there is a bit mapping technique known as Trellis Coding to prevent interference from causing sequential errors in the data stream. The addition of Trellis Coding also provides controlled redundancy by doubling the number of signal points and, interference will not cause sequential errors, because the bits are re-coded in an order that will prevent sequential errors. In the case of 256-QAM, coding gain due to Trellis Coding is a factor of 5.5 or a 7 dB increase in SNR.
The combination of RS Coding and Trellis Coding provides a maximum of 12 dB increase in SNR. This improvement becomes very significant when loop length becomes greater than 12,000 feet or when the external interference is distorting the bitstream. The downside of using both of these error correction schemes simultaneously is an increase in overhead of 15% to 20%. Therefore, when both techniques are operating, the actual throughput (worst case) of a 6.144 Mbit/sec system is 4.915 Mbits/sec. This is the required overhead to correct an errant symbol. The alternative, of course, is to re-transmit every time there is a symbol error, which would result in a 50% (worst case) reduction in throughput. In reality, the typical DMT system transmits more than 8 bits/symbol to maintain, under most conditions, a 6.144 Mbit/sec rate. If an interfering signal degrades one or more cells beyond use, the system can adjust by eliminating cells and decreasing the bit rate at 32 kbit/sec per reduction.
In a DMT design, the frequency band is split into 256 channels of 4.3125 kHz size. As can be seen in FIG. 5, the channels utilized by the Upstream data are from 26 kHz to 134 kHz, and from 138 kHz to 1.104 MHz for the Downstream portion.
Since each sub-carrier can transmit symbols (at the frequency of its sub-carrier cell), the sum of the symbols is a rate of 6.144 Mbits/sec when 256-QAM is used. In FIG. 6, the DMT Symbol Buffer 618 and the mapper 620 are the devices that code each QAM symbol and transmit it to one of the cells in the DMT Modulator 600. Each of these signals is passed to the IFFT 622, which performs an Inverse Fast Fourier Transform (IFFT) algorithm. This transforms the QAM signals into 256 symbols and passes them to the parallel in serial out module (PISO) 624. The PISO 624 module converts the bitstream from parallel to serial and then passes the signal to the DAC 628 to perform a digital to analog conversion before passing the signal through the low pass filter 630. This output is then transmitted to the receiving Modem via a line driver 630, using, most often, an Automatic Gain Circuit (AGC) to adjust the signal level based on the specific line length.
The “Web Proforum” ran a series of tests on DMT to determine what level of degradation would occur, in the presence of various types of crosstalk. The upstream signal under test was operating at 272 kbit/sec on a 26 kft local loop. The interfering signals consisted of HDSL, T1 AMI, ISDN and an SDSL signal at 784 kbits/sec. The T1 AMI signal had only a small effect on the test signal while the HDSL and SDSL signals reduced the signal reach to 12 kft or a 54% reduction. What is interesting about this result is that the DMT signal was affected by the SDSL signal as much as the CAP signal was. This indicates that neither design is a real solution to maintaining system throughput in the presence of these interfering signals.
Referring to FIG. 7, in a typical DMT system, there are 256 frequency cells 702 each of which is 4.3125 kHz wide. The highest frequency 704 being 1.104 MHz and the lowest frequency 708 being 26 kHz.
FIG. 8 shows a schematic for an embodiment of a DMT transmitter 800. The binary data stream 802 is fed into an encoder 804, which may generate CRC codes, and may serve as an interleaver, scrambler, forward error correction encoder, and/or integer-to-bit converter. A mapper 808 may assign a pre-determined number of bits to each of 256 Inverse Fast Fourier Transforms (IFFT) 810 which perform the QAM function and then pass the output to an Analog line driver 812 to send the signal to the transmission line. The number of bits assigned to a given IFFT may be determined by the noise level that is measured or predicted for that frequency. Thus, high-noise areas (such as high frequency regions, or frequencies that correspond to known interference sources, such as T1 lines) can be assigned fewer bits than low-noise areas.
It is the QAM function that creates the DMT bit stream and modulates the different frequency cells being transmitted across the Local Loop (telephone line) between the customer and the Central Office (CO).